Silicones, particularly polymers and copolymers of dimethylpolysiloxane, are used in numerous medical devices because of the known compatibility of the materials to living tissue. However, a drawback for the wider usage of silicone in the medical field is that it is expensive, and of relatively low tensile strength when compared with many other flexible plastic materials.
Accordingly, there is a need for stronger, less expensive materials than silicone; which at the same time exhibit equivalent tissue compatibility and non-clotting characteristics to silicone materials so that they may be used to make, for example, intravenous catheters designed for long-term retention in position of communcation with a blood vessel for the administration of parenteral solutions and the like.
Graft polymers including silicone materials are known, as described for example in U.S. Pat. No. 3,865,897. In this patent, graft polymers of silicone gums with polyethylene, polymethylpentene, polypropylene, and a copolymer of tetrafluorethylene repeating units and ethylene repeating units are disclosed, along with the teaching of how to graft them by placing them under shearing mixing conditions at elevated temperatures.
More details with respect to silicone-polyethylene blends are disclosed in the article entitled Silicone Polyethylene Blends by James R. Falender, et al in Polymer Engineering and Science, Vol. 16, No. 1, Jan., 1976. However, the materials of this type which have been tested have not exhibited satisfactory melt processability to get a smooth surface, or the desired higher ultimate strength and elastic behavior which would be desired for an intravenous catheter or a blood bag, for example.
Also, it is usually desired for intravenous catheters, in particular, to be radiopaque for purposes of visualization of the catheter on an X-ray fluoroscope or the like. However, silicone catheters which contain enough of a radiopaque filler such as barium sulfate to be strongly radiopaque may be of very low tensile strength. Polytetrafluoroethylene catheters and catheters made of many other materials which contain a high level of such a radiopaque filler, become unduly stiff.
Accordingly, there is a need for a flexible, plastic material which is relatively non-clotting and highly tissue compatible, and which can also remain flexible and strong at a high loading of radiopaque filler, for example on the order of 10 to 50 percent by weight.
The formulations of this invention exhibit good compatibility with tissue, reducing the normal irritation of tissue in the presence of a foreign material. Also they have low thrombogenicity, and thus can be used in longer-term contact with blood. Furthermore, the materials of this invention can remain relatively flexible and strong, when compared with other medical materials, even when loaded with extraordinarily high amounts of a radiopaque filler such as barium sulfate. Accordingly, tissue-compatible catheters for long-term implantation, and particularly, radiopaque catheters are advantageously made by this present invention.